The present invention generally relates to a current-sensitized transfer type printer, and particularly to a current-sensitized transfer type printer in which voltage is supplied to a resistance layer of an ink sheet, which has a resistance layer, a conductive layer and an ink layer, so that a fused ink is transferred to a recording sheet.
A current-sensitized transfer type printer is disclosed in Japanese Pat. Laid Open Publication No. 63-7952. This conventional current-sensitized transfer type printer is, for example, shown in FIG.1. Referring to FIG.1, the current-sensitized transfer type printer has an ink sheet 1. The ink sheet 1 comprises an ink layer 2, a conductive layer 3 and a resistance layer 4, which are stacked in this order. When a printing process for forming an image on a recording sheet 10 is carried out, the ink layer 2 is maintained in a condition in which the ink layer 2 is in contact with a recording sheet 10. A plurality of recording electrodes 5 and a common electrode 6 are respectively in contact with the resistance layer 4 of the ink sheet 1.
One or a plurality of recording electrodes 5 are selected in accordance with a printing signal corresponding to image data, and then a voltage output from a power supply 7 is applied across each selected recording electrode 5 and the common electrode 6. When the voltage is applied across each recording electrode 5 which is selected in accordance with the printing signal and the common electrode 6, a current flows through a corresponding portion of the resistance layer 4, each selected recording electrode 5 being in contact with each corresponding portion. Thus, each of the corresponding portions of the resistance layer 4 through which the current flows generates heat, so that heated ink of the ink layer 2 is fused or sublimated and transferred to the recording sheet 10.
In general, the current-sensitized transfer type printer has the following disadvantage.
That is, when the number of the recording electrodes 5 which are selected in accordance with the printing signal changes from the number of the same previously selected, the amount of current which flows into each selected recording electrode 5 changes. Therefore, the density of each dot, in an image, formed by each corresponding recording electrode 5 changes in accordance with the printing signal.
The above disadvantage occurs due to the following two causes.
First, a contact resistance between the common electrode 6 and the resistance layer 4 of the ink sheet 1 changes in accordance with the number of the selected recording electrodes 5.
The resistance layer 4 generally has a rough surface, as shown in FIG.2, with which the common electrode 6 is in contact. Thus, in this state, the contact resistance between the resistance layer 4 and the common electrode 6 is relatively large. In a case where the number of the selected recording electrodes 5 is small, the amount of the current flowing through the resistance layer 4 is small. As a result, the state where the contact resistance between the resistance layer 4 and the common electrode 6 is large is maintained, so that the current flowing into each selected recording electrode 5 is small. On the other hand, in a case where the number of the selected recording electrodes 5 is large, the amount of the current flowing through the resistance layer 4 is large, so that the amount of heat generated by the resistance layer 4 increases and the resistance layer 4 is softened. As a result, an area where the resistance layer 4 and the common electrode 6 are in contact with each other increases, so that the contact resistance between the resistance layer 4 and the common electrode 6 decreases. When the contact resistance between the resistance layer 4 and the common electrode 6 decreases, the current flowing into each selected recording electrode 5 increases.
Second, a voltage, V.sub.R ' supplied to each portion of the resistance layer 4, each portion being in contact with each corresponding selected recording electrode 5, greatly changes in accordance with the number of the selected recording electrodes 5.
An equivalent circuit substantially representing a system including the resistance layer 4, the recording electrodes 5, the common electrode 6 and the power supply 7 is illustrated in FIG.3. In FIG.3, R denotes the contact resistance between the resistance layer 4 and the common electrode 6. R denotes a resistance of each corresponding portion of the resistance layer 4, each selected recording electrode 5 being in is contact with each corresponding portion. The resistance layer 4 of the ink sheet 1 has the rough surface as shown in FIG.2, so that the contact resistance R between the resistance layer 4 and the common electrode 6 is relatively large. In a case where the contact resistance R is large as described above, when the number of the selected recording electrodes 5 changes from the number of the same previously selected and a combined resistance (R'=r/n where n is the number of selected recording electrodes 5) of corresponding portions of the resistance layer 4, each corresponding portion having a resistance R, the drop voltage V.sub.R at the contact resistance R greatly changes. As a result, when the number of the selected recording electrodes 5 changes, a voltage V.sub.R ' supplied to each corresponding portion of the resistance layer 4, each corresponding portion being in contact with each corresponding selected recording electrode 5 also greatly changes.
To eliminate the above disadvantage, in the conventional current-sensitized transfer type printer, the width of a current pulse supplied to each selected recording electrode 5 is controlled in accordance with the number of the selected recording electrodes 5. However, in this conventional current-sensitized transfer type printer, it is necessary to count the number of the recording electrodes 5 which the current should be supplied to. As a result, a circuit for supplying the current to the selected recording electrodes 5 becomes complex, and the cost for the circuit increases.